Conventional hermetic refrigerator compressors typically use fixed speed single phase induction motors. Variable speed operation of motors is advantageous for improving efficiency. Conventional three phase permanent magnet (PM) motors, such as those being used for heating, ventilating, and air conditioning applications, are capable of variable speed operation but are more expensive than single phase PM motors which require fewer power semiconductor switches and associated gate drivers.
Refrigeration compressors which are hermetically sealed to prevent refrigerant leakage have several requirements of their motor drives. Most such compressors are designed to operate with a preferred direction of rotation due to the passive lubrication system which usually operates correctly in only one direction. Furthermore, a three-pin connector has been adopted as an industry standard for such compressors, so it is advantageous to have a maximum of three wires between the motor inside the compressor and its controller situated outside the compressor. The motor additionally should have long term reliability under high temperature operation (typically 65.degree. C. ambient) and be capable of maintaining output torque and efficiency at rated speed by maintaining the current in phase with the motor back electromotive force (EMF) by appropriately phase-advancing the commutation signal.
Single phase PM motors require a suitable current commutation signal synchronized with the rotor position for proper operation. In most single phase applications, a Hall-effect position sensor is typically used to detect the rotor position and thereby control the motor. Such single phase motors having a Hall-effect sensor, however, generally require a total of five wires: two motor leads and three leads for the Hall-effect sensor (two too many for the standard sealing connector). Furthermore, the reliability of such sensors in the compressor environment is uncertain.
In order to avoid the use of a Hall sensor or other rotor position sensor, various sensorless control schemes have been developed for PM motors. In three phase PM motors under normal operation, there are times when one phase is open-circuited and has no current flowing in it. Under such conditions, the terminal voltage is equal to the back EMF voltage and can thus be sensed directly. Single phase motors, however, do not have natural intervals where the phase current remains zero for any length of time, and this approach is therefore not applicable.
For three phase motors, even if the phase current is non-zero, the back EMF voltage can be calculated by modeling the motor as a resistance, inductance, and back EMF voltage source, as described by M. Jufer, "Back-EMF Indirect Detection for Self-Commutation of Synchronous Motors," European Power Electronics Conference, 1987, pp. 1125-29. This technique can also be applied to single phase PM motors and has the advantage of not requiring any extra sensing leads. However, for single phase PM motors, it is difficult to provide a controllable preferred direction of rotation. Thus, the motor can start in either direction, depending on the initial rotor angular position. Fan and compressor drives generally are designed to operate in only one direction of rotation, so control over the rotation direction is critical. Furthermore, the required knowledge of the motor parameters is not always available and is subject to production and operating variations.